1. Field of the Invention
The invention relates to a device for fixing a heat distribution covering on a printed circuit board having a plurality of heat-generating electronic components arranged on a placement area of the printed circuit board.
Heat distribution coverings of this type serve to take up the heat from the heat-generating electronic components and to distribute the heat energy over a relatively large cooling area. However, the fixing of a heat distribution covering of this type constitutes a problem since corresponding fixing elements on the one hand occupy part of the placement area and on the other hand impair the strength of the printed circuit board. Furthermore, the area losses arising as a result of each fixing element multiply, in the case of a multilayer printed circuit board, with the number of conduction planes provided.
It is accordingly an object of the invention to provide an improved device for fixing a heat distribution covering, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and further a circuit board provided with a heat distribution covering that is attached with the improved device.
With the foregoing and other objects in view there is provided, in accordance with the invention, a device for fixing a heat distribution covering on a printed circuit board. The printed circuit board (PCB) has a placement area carrying a plurality of heat-generating electronic components and at least one conductor track plane for connecting to contact terminals of the components. The fixing device comprises:
a fixing foot attached on the placement area above at least one further conductor track plane extending below the fixing foot; and
a fixing element disposed on the fixing foot and configured to engage and hold the heat distribution covering on the printed circuit board.
Specifically, the fixing foot is disposed to be aligned opposite an opening formed in the heat distribution covering.
In other words, the invention provides for a device for fixing a heat distribution covering on a printed circuit board with a plurality of heat-generating electronic components arranged on a placement area of the printed circuit board and has at least one conductor track plane for interconnecting contact connections of the components.
In a region on the placement area which has at least one fixing foot, the fixing foot bears a placement element. The placement element can be brought into engagement with the heat distribution covering and at least one further conductor track plane is arranged below the region of the fixing foot.
This type of fixing has the advantage that it is only for the at least one fixing foot on the placement area of the printed circuit board that a corresponding metallic layer, of which the conductor tracks are also composed, has to be uncovered in order to arrange and fix the fixing foot on this metallic area. There is no need to fit further mechanical clamping or fixing devices through the printed circuit board itself, so that, in particular, in the case of multilayer printed circuit boards, conductor track planes lying below the region which is occupied by the fixing foot are available with no loss of area for the conductor track routing. Consequently, the device according to the invention provides the possibility of exhausting the full volume of the printed circuit board for the conductor track connections and limits the area requirement on the printed circuit board only to the fixing area required for at least one fixing foot on the topmost conductor track plane of the printed circuit board.
In accordance with an added feature of the invention, the device has a soldered connection between the fixing foot and the placement area, the soldered connection having the advantage that it can be combined temporarily with soldering connecting steps for fitting the electronic components, thereby obviating additional processing steps.
In accordance with an additional feature of the invention, the device has a friction welding connection between the fixing foot and the placement area. The friction welding connection has the advantage that it is only locally in the region of the fixing foot that the printed circuit board is heated during the production of the connection and the induction of thermoelectric voltage in the printed circuit board is reduced compared with a soldered connection.
Furthermore, the device may have an adhesive bonding between the fixing foot and the placement area. Since an adhesive bonding connection can be realized without a temperature increase in this case, a thermally nonstressed connection results, which is advantageous in many applications.
In accordance with another feature of the invention, the heat distribution covering has an opening arranged opposite the fixing foot. This embodiment is associated with the advantage that fixing elements can be inserted through the opening arranged opposite the fixing foot and can thus bring the heat distribution covering into engagement with the fixing foot.
To that end, in one embodiment of the invention, the fixing element may be a threaded pin applied orthogonally to the placement area on the fixing foot. A fitting nut can be screwed onto a thread pin of this type, the nut being arranged with an accurate fit in the opening in the heat distribution covering, so that the heat distribution covering is in engagement with the fixing foot via the threaded pin with the aid of the fitting nut reaching through the opening. This results in a releasable fixedly anchored fitting of the heat distribution covering on the printed circuit board.
In a further embodiment of the invention, heat-generating components on the printed circuit board whose structural height is smaller than the distance between heat distribution covering and printed circuit board are thermally conductively connected to the heat distribution covering by means of a heat-conducting filling material which is arranged between the individual electronic component and the heat distribution covering. This embodiment of the invention has the advantage that a wide variety of heat-generating components with varying structural height can be cooled by the heat distribution covering by means of the heat-conducting filling material.
Instead of a threaded pin applied orthogonally to the placement area on the fixing foot, it is also possible to arrange a fixing element with an internal thread, such as a fixing nut, on the fixing foot. In this case, the heat distribution covering is screwed to the internal thread of the fixing element with the aid of a covering screw which projects through the opening in the heat distribution covering. Both a fixing nut and a threaded pin may be formed in one piece with the fixing foot, thereby reducing the number of components to be stored for the fixing according to the invention.
In a further embodiment of the invention, the fixing element is a plug pin with an attachment via which a plug element arranged with an accurate fit in the opening in the heat distribution covering is in engagement. A plug element of this type may have, in the region of the attachment of the plug pin, snap-action hooks which slide over the attachment and spread apart and then latch into place behind the attachment, so that the heat distribution covering is fixedly connected to the fixing foot and thus to the printed circuit board via the plug element and the plug pin. The plug pin and the fixing foot may be formed in one piece, which again affords the advantage that the number of fixing elements to be stored is reduced.
In a further embodiment of the invention, the fixing element is a cylindrical wherein is arranged orthogonally to the placement area of the printed circuit board on the fixing foot or is formed in one piece with the fixing foot. A cylindrical pin of this type as fixing element may project through the opening in the heat distribution covering and the heat distribution covering may be fixed at the cylindrical pin by means of a crown disk. The radially inner cutting areas of the crown disk are anchored in the material of the cylindrical pin in this case.
Furthermore, the opening itself may have the form and the cutting areas of a crown disk, so that the heat-distributing covering can be placed directly onto the cylindrical fixing pin and anchors itself through the correspondingly formed openings at the cylindrical pin. This embodiment has the advantage that, apart from a fixing pin with fixing foot, no further fixing elements are necessary for anchoring the heat distribution covering on the printed circuit board. Forming the openings with cutting areas similar to a castle nut can be achieved by stamping pressing in one work operation with a suitably designed stamping tool.
In a further embodiment of the invention, the fixing element may be a rivet-type wherein, after the application of the heat distribution covering, is reshaped to form a rivet head at its free end projecting through the opening in the heat distribution covering. However, the pin may also be designed as a hollow rivet, so that only the edge of the hollow rivet which protrudes through the opening in the heat distribution covering is to be flanged.
In a further embodiment of the invention, the printed circuit board may have a soldering resist layer on both surfaces and the heat-generating electronic components may be arranged on the placement area on the printed circuit board using CSP technology (CSP, chip scale package), LFBGA technology (LFBGA, low-profile fine-pitch ball-grid array) or using flip-chip technology. Processes in flip-chip technology, in particular, requires a multilayer printed circuit board in order to be able to interconnect the multiplicity of contact areas of the individual heat-generating electronic components.
In particular, the invention thus relates to the fixing of heat spreaders (or heat distribution coverings) on memory modules in such a way that as little space as possible on the printed circuit board (PCB) has to be used for the fixing. A problem in the case of high-density memory modules is the space requirement of the memory and peripheral components and also the electrical connection thereof by routing of the conductor tracks on the PCB. The difficulty is that the fixing points for the heat spreader cannot be chosen freely, but rather must be present in a defined arrangement and number in order to achieve a sufficiently good fixing and a good contact of the gap filler (or heat-conducting filling material which conducts the heat from the components (or electronic components) into the heat spreader and is intended to compensate for mechanically small height differences between the components).
Fixing with hollow rivets requires bored holes in the PCB. It is not possible to route the conductor tracks in the region of the holes and a certain safety zone (keep out zone), as a result of which, in the case of high-density memory modules, it may become impossible to connect the components by means of conductor tracks. The xe2x80x9ckeep out zonexe2x80x9d may additionally be enlarged by the fact that the heat spreader (HS) is embossed, as a result of which it is necessary additionally to comply with a safety clearance between the components and the embossing zone.
According to the invention, a fixing element is mounted on the PCB. This element performs the fixing of the heat spreader on the PCB. In one embodiment of the invention, the fixing element is placed together with the components on the PCB, i.e. ideally fixing elements with xe2x80x9cSMD capabilityxe2x80x9d are involved which are emplaced by the placement machine and fixed during soldering (SMD, surface-mounted device).
The fixing of the element is done by soldering by means of SMD soldering paste, since it is thus not necessary to use any further materials. As an alternative, other connections between placement areas of the printed circuit board and fixing foot of a fixing element, such as an adhesive connection, a laser welding connection or a friction welding connection are also suitable for particular embodiments.
For soldering, the size of the land pad, the geometry of the fixing foot of the fixing element and also the geometry of the soldering resist mask (solder mask) are coordinated with one another in such a way that the fixing element as far as possible has a self-centering action during soldering, so that it is drawn into the desired position by the surface tensions acting during soldering. Furthermore, the geometry of the fixing element with fixing foot is set in such a way that, in principle, SMD placement is possible, hereby ensuring processability on automatic placement machines, with the element securely situated on the PCB without xe2x80x9cfalling overxe2x80x9d during the processing.
With the above and other objects in view there is also provided, in accordance with the invention, a printed circuit board assembly, comprising:
a printed circuit board having a placement area carrying a plurality of heat-generating electronic components having respective connection terminals;
at least one conductor track plane for interconnecting said connection terminals of said electronic components;
a device for fixing a heat distribution covering on said printed circuit board, said device including:
a fixing foot attached on said placement area above at least one further conductor track plane extending below said fixing foot; and
a fixing element disposed on said fixing foot and configured to engage and hold the heat distribution covering on the printed circuit board.
With the above and other objects in view there is also provided, in accordance with the invention, a printed circuit board assembly, comprising:
a printed circuit board having a placement area carrying a plurality of heat-generating electronic components having respective connection terminals;
a first conductor track plane interconnecting said connection terminals of said electronic components, and a second conductor track plane below said first conductor track plane;
a heat distribution covering disposed to cover said electronic components; and
a device for fixing said heat distribution covering on said printed circuit board, said device including a fixing foot attached on said placement area above said second conductor track plane; and a fixing element disposed on said fixing foot and engaging and holding said heat distribution covering on said printed circuit board.
The invention thus has a plurality of advantages, which include, by way of example:
On the PCB, only the topmost conductor track plane is used for the fixing element; the central layers can still be used for the routing of conductor tracks. In other words, the invention affords a high degree of flexibility for the PCB design.
At the same time, deep embossing of the heat spreader in the region of the fixing element becomes superfluous or the heat spreader can be made flatter since embossing only becomes necessary if very narrow thickness restrictions apply to the memory module, if e.g. a rivet head is to be countersunk in an embosser. As a result, the components can also be positioned nearer to the fixing element.
The invention can be used both for single-sided and for double-sided HS mounting.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a device for fixing a heat distribution covering on a printed circuit board, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.